But through the magic of bar codes, and computers, USPS and Netflix have worked out a work-around. For the past few weeks, in addition to the 4 DVD's for which we pay, we see a little note about an additional DVD in the mail.
I wrote "guess" above, but it's confirmed by a comment in a reddit thread.
What's totally mystifying to me is that I don't see any publicity from Netflix. Maybe they or their USPS partner are worried whether someone will challenge the arrangement?